At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminance LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, in which the white light is created by mixing the light of the near ultraviolet/ultraviolet to blue color generated from the LED and the light generated from the phosphor having an excitation band in a wavelength region thereof. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of the LED and a phosphor, the white LED has advantages of good life span without burn-out of a filament like a conventional incandescent bulb and the harmful substance such as mercury is not contained, and miniaturization of the illumination device is realized, thus realizing an ideal illumination device.
Two systems are proposed as the system of the LED illumination. One of them is a multi-chip type system which creates white color by using three primary color LEDs such as high luminance red LED, green LED, and blue LED, and the other one is one-chip type system which creates white color by combining a high luminance LED emitting light of near ultraviolet/ultraviolet to blue color and a phosphor excited by the light of the near ultraviolet/ultraviolet to blue color emitted from this LED. When these two systems are compared from the viewpoint of illumination, particularly in the one-chip type system, the phosphor having an emission spectrum with a broad peak is used, therefore the emission spectrum can be made closer to the spectrum of solar light, and therefore the white light having excellent color rendering properties can be obtained, compared to the multi-chip type system. Further, the one-chip type system has many advantages such as a simplified drive circuit, enabling miniaturization, eliminating an optical waveguide for mixing colors, with no necessity of considering a difference in drive voltage and optical output of each LED, and reducing a cost. Therefore, the one-chip type system, in which an LED and a phosphor are combined, is focused as an illumination of the next generation.
The white LED illumination, in which a high luminance blue LED and a phosphor emitting yellow color by being excited by the blue light generated from the LED are combined, is given as one of the examples of the one chip type white LED illumination. Specifically, for example, a high luminance blue LED and an yellow phosphor (Y,Gd)3, (Al,Ga)5O12:Ce(YAG:Ce), Tb3Al5O12:Ce, Ca3Sc2Si3O12:Ce, and CaSc2O4:Ce can be combined. In the white LED illumination, white color is obtained by using a complementary relation between the blue emission of the LED and the yellow emission of the phosphor, thereby allowing less phosphor to be used. Further, the yellow phosphor YAG:Ce used for the white LED illumination has an excitation spectrum with a peak near the wavelength of 460 nm, thereby allowing emission with high efficiency, and has an emission spectrum with a luminance (visibility) peak at about 560 nm, thereby allowing high luminance white LED to be obtained. However, the problem of the white LED illumination is that the emission on the longer-wavelength side of visible light range, specifically the emission of red color component is insufficient, and therefore, only slightly bluish white emission can be obtained, and a slightly reddish white emission like an electric bulb can not be obtained, thereby the color rendering properties are insufficient. However, in recent years, a nitrogen-containing phosphor having a broad emission spectrum with a peak in a wavelength range from yellow color to red color, and also having a good excitation band in a range from near ultraviolet/ultraviolet to blue color has been developed in sequence. Then, by adding such a phosphor, the color rendering properties are improved.
Also, another example of the one chip type white LED illumination obtains white color by using a mixed color of lights of the LED emitting the near ultraviolet/ultraviolet color, and lights of the phosphor emitting red color (R), the phosphor emitting green color (G) and the phosphor emitting blue (B) color, obtained by being excited by the near ultraviolet/ultraviolet light generated from the LED. A method of obtaining white emission by the lights of the R, G, B, and other colors is capable of obtaining an arbitrary emission color other than white light, depending on the combination and mixed ratio of the R, G, B, and is excellent in color rendering properties, because the white emission is obtained not by the complementary relation of the lights but by the relation of mixed colors using the R,G,B.
Then, as the phosphor used for such an application, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu for the red phosphor, ZnS:Cu,Al, CaGa2S4:Eu, SrGa2S4:Eu, BaGa2S4:Eu, SrAl2O4:Eu, BAM:Eu,Mn, (Ba,Sr,Ca,Mg)2SiO4:Eu for the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu for the blue phosphor. However, the red phosphors out of the phosphors of three colors have a sharp emission spectrum, while the phosphors of other colors have broad emission spectra, thereby involving the problem that the color rendering properties of the white light obtained is unsatisfactory, and emission characteristic at a high temperature is deteriorated. However, such a problem has also been solved, as described above, by developing in sequence phosphors containing nitrogen, excellent in temperature characteristic and excitation band characteristic, and emitting from yellow color to red color.
The problem involved in the phosphor emitting yellow color to red color is substantially solved, by developing the nitrogen-containing phosphor having the emission spectrum with a peak in the wavelength range from yellow color to red color, having a broad emission spectrum, and further having a good excitation band in the wavelength range from the near ultraviolet/ultraviolet to blue color. As the phosphor containing nitrogen as described above, Ca2Si5N8:Eu, Sr2Si5N8:Eu, Ba2Si5N8:Eu, Cax(Al,Si)12(O,N)16:Eu (0<x≦1.5), CaAl2Si4N8:Eu, CaAlSiN3:Eu and so forth are typically given as examples.
Here, as a necessary factor as a light source for a general illumination such as a white LED as described above, firstly a factor of brightness and secondary a factor of color rendering properties are given. As the first factor of brightness, the brightness (luminance) as the light source and emission efficiency are given, and in the LED, are largely influenced by the emission efficiency of the used semiconductor element, the emission efficiency of the used phosphor, and the structure of the white LED itself. As the second factor, the color rendering property, a value showing reproducibility of the color by the light source is given as an example, and generally an evaluation method of this color rendering property is shown in JISZ8726 (1990). Therefore, the color rendering property will be explained by using the evaluation method of JISZ8726.
According to JISZ8726, the color rendering property of the light source is numerically expressed by a general color rendering index (Ra). This is a value obtained by evaluating a difference of colors between a reference sample for color rendering evaluation illuminated by a sample light source, and the reference sample illuminated by a reference light approximated a natural light, and when there is no difference between them and they are completely the same values, the color rendering index (Ra) is 100. Even if color temperatures of the light sources are the same, there is a difference in the way color is observed depending on the color rendering index, and when the color rendering index is low, a color is dull and appears dark. When the light source has a uniform density of light over an entire region of visible light, this light source has an excellent color rendering property.
The color rendering property is improved by development of the aforementioned new phosphor emitting light from yellow color to red color, and the next problem is the phosphors with the emission peak wavelength from green color to yellow color.
First, the problem involved in the yellow phosphor YAG:Ce is explained by using FIG. 25. FIG. 25 is a graph showing the emission intensity (relative intensity) taken on the ordinate axis and the wavelength of an excitation light taken on the abscissa axis, and showing an excitation spectrum obtained by measuring an intensity of the light having the wavelength of 559.2 nm emitting light when the YAG:Ce is excited by an excitation light having the wavelength of 300 to 570 nm.
In the white LED illumination obtained by combining the high luminance blue LED and the YAG:Ce phosphor emitting yellow color by being excited by blue color generated from the LED, the YAG:Ce phosphor has a high-efficiency excitation band for the light having the wavelength of 460 nm generated from the blue LED, and further, has an emission spectrum with a peak near to the wavelength of 560 nm in which the luminance (visibility) is highest, thereby allowing a high luminance white LED illumination to be obtained. However, as clarified from FIG. 25, the YAG:Ce phosphor has an emission characteristic of emitting the light having the wavelength of 560 nm or around with high efficiency, when excited by the light having the wavelength of 460 nm. However, since the excitation band is narrow, the emission wavelength of the blue LED changes due to variation in manufacturing the blue LED, then if the emission wavelength is deviated from the range of an optimal excitation band of the YAG:Ce phosphor when excited by the blue light of the blue LED, disruption of balance between the blue color and yellow color emission intensity occurs. Such a situation involves the problem that color tone of the white light obtained by combining the blue light and the yellow light is changed.
Further, this YAG:Ce phosphor has an excellent emission spectrum in the wavelength range from about 500 to 550 nm of green color component of visible light. Therefore, preferably the YAG:Ce phosphor is used as a green phosphor of the white LED illumination in which the near ultraviolet/ultraviolet LED, the red (R) color emitting phosphor, the green (G) color emitting phosphor, and the blue color (B) emitting phosphor are combined. However, when light-emitted by the near ultraviolet/ultraviolet light, as shown in FIG. 25, this YAG:Ce phosphor has a low-efficiency excitation band in the emission wavelength (380 to 410 nm or around in FIG. 25) of the near ultraviolet/ultraviolet LED. Therefore, the problem involved therein is that a sufficient emission can not be obtained, and the high luminance white LED illumination can not be obtained.
Next, the problem involved in the green phosphor used in combination with the ultraviolet LED will be explained. As the white LED illumination using the near ultraviolet/ultraviolet emitting LED and the mixed colors of light of the red (R) color emitting phosphor, the green (G) color emitting phosphor, and the blue (B) color emitting phosphor obtained by being excited by the light of the near ultraviolet/ultraviolet light generated from the LED, the green phosphor such as ZnS:Cu,Al, SrAl2O4:Eu, BAM:Eu,Mn, (Ba,Sr,Ca,Mg)2SiO4:Eu are presently used. Among the phosphors, there is a problem that a sulfide phosphor is significantly deteriorated in emission intensity, when heat is applied thereto, and further has no water-resisting property. In addition, an oxide phosphor does not have a good efficient excitation band in a broad range of the wavelength in the vicinity of the near ultraviolet/ultraviolet. Therefore, the problem involved therein is that when the variation in emission wavelength occurs due to by variation in manufacturing the near ultraviolet/ultraviolet LED, the emission wavelength of the near ultraviolet/ultraviolet LED is deviated from the optimal excitation range, thereby disrupting the balance in emission intensity among the red color, green color, and blue color, resulting in the change of the color tone of the white light.
Therefore, as the green to yellow emitting phosphor by being excited by the light of the near ultraviolet/ultraviolet to blue color also, demand on a new phosphor having a flat high-efficiency excitation band in the wavelength range from near ultraviolet/ultraviolet to blue color, and having a broad emission spectrum, and further having an excellent durability against heat and water, and replacing YAG:Ce phosphor and ZnS:Cu,Al phosphor is increased. In order to respond to such a demand, green to yellow emitting phosphors are actively pursued, and in recent years, silicon nitride-based phosphor (for example see patent document 1), a phosphor comprising sialon as a matrix (for example, see patent documents 2, 3, 4), and oxynitride phosphor (for example, see patent documents 5 and 6) are proposed as the green to yellow emitting phosphor.    (Patent document 1) Japanese Patent Laid Open No. 2002-322474    (Patent document 2) Japanese Patent Laid Open No. 2003-203504    (Patent document 3) Japanese Patent Laid Open No. 2003-206481    (Patent document 4) Japanese Patent Laid Open No. 2002-363554    (Patent document 5) WO Publication No. 2004/029177 A1 pamphlet    (Patent document 6) WO Publication No. 2004/055910 A1 pamphlet